linux-kernel - Re: [PATCH] KVM/x86: Do not clear SIPI while in SMM

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Date: Tue, 16 Apr 2024 15:14:53 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Paolo Bonzini <pbonzini@...hat.com>
Cc: boris.ostrovsky@...cle.com, kvm@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM/x86: Do not clear SIPI while in SMM

On Wed, Apr 17, 2024, Paolo Bonzini wrote:
> On Tue, Apr 16, 2024 at 10:57 PM <boris.ostrovsky@...cle.com> wrote:
> > On 4/16/24 4:53 PM, Paolo Bonzini wrote:
> > > On 4/16/24 22:47, Boris Ostrovsky wrote:
> > >> Keeping the SIPI pending avoids this scenario.
> > >
> > > This is incorrect - it's yet another ugly legacy facet of x86, but we
> > > have to live with it.  SIPI is discarded because the code is supposed
> > > to retry it if needed ("INIT-SIPI-SIPI").
> >
> > I couldn't find in the SDM/APM a definitive statement about whether SIPI
> > is supposed to be dropped.
> 
> I think the manual is pretty consistent that SIPIs are never latched,
> they're only ever used in wait-for-SIPI state.

Ya, the "Interrupt Command Register (ICR)" section for "110 (Start-Up)" explicitly
says it's software's responsibility to detect whether or not the SIPI was delivered,
and to resend SIPI(s) if needed.

  IPIs sent with this delivery mode are not automatically retried if the source
  APIC is unable to deliver it. It is up to the software to determine if the
  SIPI was not successfully delivered and to reissue the SIPI if necessary.